1. Field of the Invention
The present invention relates to a web charging apparatus that deposits electrostatic charges on a moving web and is provided preceding to a coating apparatus, which coats the web with a variety of coating solutions in order to manufacture photographic film, photographic printing paper, printing photosensitive materials, medical photosensitive materials, microfilm, magnetic recording tape, adhesive tape, pressure sensitive paper, thermal paper, offset printing plate materials, etc.
2. Description of the Related Art
U.S. Pat. No. 5,138,971 which corresponds to Japanese Patent Provisional Publication No. 4-65088 discloses that a web charging apparatus is provided preceding to a coating apparatus, so as to improve the affinity and adhesion of the coating solution to the web by depositing unipolar electrostatic charges on the surface of the web before the coating solution is applied on the surface. In the web charging apparatus, corona discharge is established between wire electrodes, which are extended in a widthwise direction of the web, and the web that is supported on a grounded roller functioning as a grounded electrode relative to the wire electrodes, so that the unipolar electrostatic charges can be deposited on the web. Thereby, the coating solution can be easily applied on the web at the start of the coating, and it is also possible to prevent the coating solution from being applied too thickly. Moreover, it is also possible to prevent the coating solution from being disturbed when a web splicing part is coated. Hence, the coating method utilizing the electrostatic field has usually been adopted to a variety of coating apparatues.
Recently, the web is coated with the coating solution while moving at high speed, and it has been found that the electrostatic potential at both widthwise ends of the web is lower than the electrostatic potential at the other area of the web when the unipolar electrostatic charges are deposited on the surface of the web that moves at high speed. Both widthwise ends of the web with the low electrostatic potential will be referred to as "edge parts of the web", and the other area of the web will be referred to as "the central part of the web". For this reason, when the coating solution is applied on the web, the affinity and adhesion of the coating solution on the edge parts of the web are much worse than those on the central part of the web.
To solve the above-mentioned problem, U.S. Pat. No. 5,138,971 discloses the web charging apparatus in which the distance between the wire electrode and each edge part of the web is shorter than the distance between the wire electrode and the central part of the web.
In the above-mentioned case, however, the distance between the wire electrode and the web must be finely adjusted by less than 1mm. Even if the distance between the wire electrode and the web can be adjusted as desired, the wire electrode easily sags, and thereby, the electrostatic potential at the edge parts of the web becomes lower than the electrostatic potential at the central part of the web. Thus, so-called "liquid exhaustion at the edge parts of the web" arises in that the edge parts of the web cannot satisfactorily be coated compared with the central art of the web.